Optical Coherence Tomography (OCT) is a non-invasive, interferometric optical imaging technique that can generate micron resolution 2D and 3D images of tissue and other scattering or reflective materials. OCT is often used for biomedical imaging or materials inspection. First demonstrated for imaging the human eye and coronary arteries in 1991, OCT has since been established as a clinical standard for diagnosing and monitoring treatment of eye disease. OCT is also used for intravascular imaging of plaque to assess heart disease, cancer biopsy imaging, developmental biology research, art preservation, industrial inspection, metrology, and quality assurance. In general, OCT is useful for applications that benefit from subsurface imaging, surface profiling, motion characterization, fluid flow characterization, index of refraction measurement, birefringence characterization, scattering characterization, or distance measurement.
Optical Coherence Tomography uses the interference pattern obtained by combining light backscattered or backreflected from a sample with light from a reference arm to determine spatially dependent properties of the sample, as illustrated in FIG. 1A. Time Domain OCT (TD-OCT) imaging principles were used in the first demonstrations and commercial products for OCT. However, TD-OCT is known to be a slow technology for acquiring OCT data. Fourier Domain OCT (FD-OCT) enables orders of magnitude faster imaging speeds than TD-OCT and has become the current research and commercial standard. Fourier Domain OCT can be implemented with a broadband light source, interferometer, spectrometer, and linescan camera, called Spectral Domain OCT (SD-OCT), as illustrated in FIG. 1B. Scanning the light across a sample (FIG. 1C) allows the collection of a complete reflectivity vs. depth profile, called an A-scan (FIG. 1D), for each point of interrogation. Scanning and assembling sequentially acquired A-scans allows 2D images to be formed, called B-scans (FIG. 1E). 3D volumes are also formed by scanning across the sample in two directions (FIG. 1F). Fourier Domain OCT can also be implemented with a wavelength swept light source, interferometer, detector, and analog to digital converter (A/D), called Swept Source OCT (SS-OCT) or optical frequency domain imaging (OFDI), as illustrated in FIGS. 2A and 2B. For the purposes of this disclosure, Swept Source OCT and OFDI are equivalent. The two variations of Fourier Domain OCT, being Spectral Domain OCT and Swept Source OCT, represent the state of the art in OCT imaging technologies.
Spectral Domain OCT suffers from an inherent and problematic loss of OCT sensitivity with increasing imaging depth, often called sensitivity roll-off, sensitivity fall-off, or sensitivity drop. The loss of OCT sensitivity with increasing depth is caused by a reduction in the interferometric fringe visibility due to limits in the spectrometer resolution, integration of multiple wavelengths over a pixel width, and inter-pixel crosstalk, as described in the papers, “Analytical model of spectrometer-based two-beam spectral interferometry,” Hu, Pan, and Rollins, Applied Optics, Vol. 46, No. 35, pp. 8499-8505, 2007 and “Improved spectral optical coherence tomography using optical frequency comb,” Bajraszewski et al. Optics Express, Vol. 16, No. 6, pp. 4163-4176, 2008.
A paper, “Fourier domain optical coherence tomography with a linear-in-wavenumber spectrometer,” Hu and Rollins, Optics Letters, Vol. 32, No. 24, pp. 3525-3527, 2007, teaches linearizing the spectral dispersion of the spectrometer in wavenumber using a specifically designed prism. The spectral linearity in wavenumber results in improvement of the fall-off of signal with imaging range inherent to spectral domain optical coherence tomography imaging. Although there is improvement, the loss of sensitivity with imaging depth is still significant, especially when used with wide spectral bandwidth sources for achieving a fine OCT axial resolution.
A paper, “Improved spectral optical coherence tomography using optical frequency comb,” Bajraszewski et al. Optics Express, Vol. 16, No. 6, pp. 4163-4176, 2008, teaches using a Fabry-Perot optical frequency comb in a Spectral Domain OCT system to reduce the depth dependent drop of sensitivity. The approach has several significant disadvantages. Insertion of the frequency comb reduces optical power levels, which compromises baseline OCT sensitivity. The approach also requires that the optical frequency comb be actively tuned and multiple spectrometer measurements performed for every A-scan in order to fill gaps in spectral data content that are filtered out by the Fabry-Perot filter. In practice, four camera exposures were shown to enable OCT imaging, which results in a significant reduction in OCT imaging speed.
Various so called “full range” or “complex conjugate” approaches have been proposed to extend the imaging range and help mitigate the problem of sensitivity roll-off associated with Spectral Domain OCT. These approaches do not fully suppress complex conjugate artifacts in the images, require considerable computation, and often require multiple acquisitions to construct each A-scan, so are not suitable for high dynamic range and high speed OCT acquisition. Further, maximum imaging speeds with Spectral Domain OCT are limited to several hundred kilohertz A-scan rate due to limits in linescan camera speeds. These inherent characteristics and deficiencies combined suggest that Spectral Domain OCT is not the technology of choice for long range, high speed, and high dynamic range imaging.
Swept Source OCT uses a wavelength swept laser as the light source and a detector with high speed A/D converter to sample the interferometric OCT signal. Sensitivity roll-off performance in Swept Source OCT is generally significantly better than Spectral Domain OCT. Swept Source OCT has also achieved higher imaging speeds and longer imaging range than Spectral Domain OCT.
Many different swept laser configurations and wavelength tuning mechanisms have been implemented for Swept Source OCT that either include a wavelength selective intracavity filter or wavelength selective laser cavity end mirror. Examples include: galvo-grating wavelength selective end mirror designs (Chinn, Swanson, and Fujimoto, Optics Letters, Vol. 22, No. 5, pp. 340-342, 1997), rotating polygon mirror-grating filter designs (Yun et al., Optics Letters, Vol. 28, No. 20, pp. 1981-1983, 2003), fiber ring lasers with intracavity wavelength selective filter (Huber et al., Optics Express Vol. 13, No. 9, pp. 3513-3528, 2005), and short cavity microelectromechanical systems (MEMS) filter based tunable lasers (WO 2010/111795 A1). In all of these swept laser designs, lasing builds from amplified spontaneous emission (ASE) as the filter is tuned such that the photon round trip time is significant, and along with cavity efficiency and filter width, define a maximum sweep speed at which the laser can be swept while still maintaining full saturation of the optical gain medium. Sweep repetition rates in the tens of kilohertz to low hundreds of kilohertz are generally possible with these technologies, but the sweep speed is still fundamentally limited due to the relatively long photon round trip time.
U.S. Patent Application No. 2006/0187537 A1 teaches a different swept source laser technology, called a Fourier Domain Mode Locked (FDML) laser. An FDML laser operates with a principle that enables higher sweep speeds. In an FDML laser, a long fiber loop is used to store the wavelength sweep and a filter is tuned in synchronization with the returning sweep wavelength, either before or after optical amplification. The FDML approach reduces the need to build up lasing from ASE to achieve high fundamental sweep repetition rates up to about 500 kHz axial scan rate. Through replicating, delaying, and multiplexing the sweep, buffered speeds up to about 5 MHz axial scan rate can be achieved for a single imaging spot (Wieser et al., Optics Express, Vol. 18, No. 14, 2010). A significant drawback of a typical FDML laser is a short a coherence length of about 4-10 mm, which significantly limits OCT imaging range.
In Swept Source OCT, sensitivity roll-off is limited by the coherence length of the wavelength tunable laser source, which is determined by the instantaneous linewidth of the laser. In all of the swept lasers describe thus far, the filter in the laser is designed to tune multiple laser longitudinal modes. As taught by International Patent Application Publication No. WO 2010/111795 A1 and Huber et al., Optics Express Vol. 13, No. 9, pp. 3513-3528, 2005, the wavelength selective filter in a traditional swept laser design spans multiple longitudinal laser modes in order to achieve high sweep rates and prevent laser power drop off and laser noise due to mode-hopping. In the case of the FDML laser, the reason for designing a relatively wide spectral filter width is related to dispersion in the fiber loop that causes a wavelength dependent round trip time, requiring the filter to be wide enough to transmit the full range of slow to fast wavelengths in the fiber loop. Regardless of the reason for needing to use a wide filter that spans multiple laser longitudinal modes, the result is a laser with a relatively wide instantaneous linewidth with compromised coherence length, OCT imaging range, and OCT sensitivity roll-off.
A paper, “Extended coherence length Fourier domain mode locked lasers at 1310 nm”, Adler et al., Optics Express, Vol. 19, No. 21, pp. 20931-20939, 2011 teaches a method to improve the coherence length of an FDML laser by adding a chirped fiber Bragg grating dispersion compensation module to improve the dispersion characteristics of the fiber loop. Improved laser coherence length to about 21 mm and the ability to use both the forwards and backwards sweeps were obtained.
In nearly all implementations to date, Spectral Domain OCT systems and Swept Source OCT systems have been designed to operate at a fixed imaging speed, fixed imaging range, and fixed OCT axial resolution. Generally, the entire OCT imaging system is optimized for a specific application.
With the introduction of high speed CMOS linescan camera technology with programmable speed and programmable active pixel count, it became possible to trade off pixel count to gain imaging speed in Spectral Domain OCT.
A paper, “Ultrahigh speed Spectral/Fourier domain OCT ophthalmic imaging at 70,000 to 312,500 axial scans per second,” Potsaid et al., Optics Express, Vol. 16, No. 19, pp. 15149-15169, 2008, teaches operating a Spectral Domain OCT system using a CMOS camera with adjustable active pixel count in different configurations to achieve: long imaging range with fine axial resolution and moderate OCT imaging speed, short imaging range with fine axial resolution at faster imaging speeds, and short imaging range with compromised axial resolution at ultrafast imaging speeds. Each configuration was optimized for sensitivity and imaging performance. A significant drawback of the approach is that the light source must be interchanged and the spectrometer rebuilt with different components for the multiple configurations and operating modes.
A paper, “High-Speed High-Resolution Optical Coherence Tomography at 800 and 1060 nm”, Povazay et al., Proceedings of SPIE, vol. 7139, pp. 71390R-1-7, 2008, teaches an OCT imaging system using a programmable CMOS camera with a fixed light source in which the number of pixels used in the camera is reduced in order to achieve higher imaging speeds by truncating the spectrum. A significant disadvantage of this method is that the spectrometer is not reoptimized to the light source bandwidth for the different operating modes, so light falls on unused pixels for the higher speed imaging configurations and there is an associated loss of OCT sensitivity.
A paper, “Ultra high-speed swept source OCT imaging of the anterior segment of human eye at 200 kHz with adjustable imaging range,” Gora et al, Optics Express, Vol. 17, No. 17, pp. 14880-14894, 2009, teaches a Swept Source OCT imaging systems using an FDML laser that trades off OCT axial resolution to gain imaging range. A disadvantage of this approach is that the FDML laser must be run at a harmonic of the sweep frequency, so the sweep repetition rate of the OCT imaging system cannot be changed without significant reconfiguration.
A new swept light source for use with Swept Source OCT has been developed that overcomes many of the above mentioned limitations associated with previous OCT technologies.
U.S. Pat. No. 7,468,997 B2 teaches a swept source optical coherence tomography system (SS-OCT) comprising a vertical cavity surface-emitting laser (VCSEL) with an integrated MEMs tunable mirror movable by electro-static deflection. A paper, “OCT Imaging up to 760 kHz Axial Scan Rate Using Single-Mode 1310 nm MEMS-Tunable VCSELs with >100 nm Tuning Range,” Jayaraman et al., Optical Society of America, CLEO Conference, pp. PDPB1-PDPB2, 2011, experimentally demonstrates the first widely tunable, single-mode 1310 nm MEMS VCSELs with >100 nm tuning range, and the first application of these VCSELs to ultra-high-speed swept source OCT imaging at axial scan rates up to 760 kHz. Unlike other swept laser sources, which use a short cavity and intra-cavity filter, VCSELs operate with a true single-longitudinal mode instead of a set of modes. The true single-longitudinal mode operation results in a long coherence length for the VCSEL technology. Further, forward and backward scans show comparable performance, in contrast to other swept sources, enabling use of both the forwards and backwards sweeps for OCT imaging.
The limited imaging speed, limited imaging range, loss of sensitivity with increasing imaging depth, and operation at a predominately fixed imaging mode of previous OCT technologies result in a compromise of OCT imaging performance and limit application of OCT technology.